Colour Management Information from

Graphic Quality Consultancy

Introduction to Colour Spaces - CIE Lab & LCH

A basic introduction to two related colour spaces which are becoming increasingly important in the world of colour reproduction. These are among the tristimulus (three-dimensional) colour spaces developed by the C.I.E.

What is the CIE?
C.I.E. is short for "Commission Internationale de l'Eclairage ", which in English is the "International Commission on Illumination".  A professional scientific organisation founded over 90 years ago to exchange information on all "matters relating to the science and art of lighting". The standards for colour spaces representing the visible spectrum were established in 1931 and have been revised more recently. There are many CIE colour spaces, more correctly known as models, which serve different purposes. They are all device independent, unlike RGB or CMYK colour spaces which are related to a specific device (cameras, scanners, presses, etc.) or material type (paper, inks, film emulsions and lighting, etc.). These RGB and CMYK spaces usually do not cover the entire visible colour spectrum or gamut. The CIE also specify lighting conditions.

The CIE LCH Colour Space or Colour Model.

This is possibly a little easier to comprehend than the Lab colour space, with which it shares several features. It is more correctly known as  L*C*H*.  Essentially it is in the form of a sphere. There are three axes; L* and C* and H°.  

The L* axis represents Lightness. This is vertical; from 0 which has no lightness (i.e. absolute black) at the bottom; through 50 in the middle, to 100 which is maximum lightness (i.e. absolute white) at the top.

The C* axis represents Chroma or "saturation". This ranges from 0 at the centre of the circle, which is completely unsaturated (i.e. a neutral grey, black or white) to 100 at the edge of the circle for maximum Chroma or saturation.

If we take a horizontal slice through the centre, we see a coloured circle. Around the edge of the circle we see every possible saturated colour, or "Hue". This circular axis is known as H° for Hue. The units are in the form of degrees°, ranging from 0° (red) through 90° (yellow), 180° (green), 270° (blue) back to  0°. 

The LCH colour model is very useful for retouching images in a colour managed workflow, using very high end editing or scanning applications such as Fuji ColourKit.  LCH is device-independent. A similar colour model is HSB or HSL for Hue, Saturation and Brightness (Lightness)which can be used in Adobe PhotoShop and other software. Technically this is "device-dependent", however it is particularly useful for editing RGB images. For example to edit a green:  Adjust the Hue angle by increasing it to make it "bluish" or by reducing it to make it "yellowish"; Increase the Saturation (Chroma) to make it "cleaner";  increase the Brightness or Lightness to make it lighter. Go on give it a try!

The CIE Lab Colour Space or Colour Model.

This is more correctly known as  L*a*b*. Just as in LCH, the vertical L* axis represents "Lightness", ranging from 0-100.  The other (horizontal) axes are now represented by a* and b*.  These are at right angles to each other and cross each other in the centre, which is neutral (grey, black or white). They are based on the principal that a colour cannot be both red and green, or blue and yellow. The a* axis is Green at one extreme (represented by -a), and red at the other (+a). The b* axis has blue at one end (-b), and yellow (+b) at the other.  The centre of each axis is 0. A value of 0 or very low numbers of both a* and b* will give a neutral or near neutral.

CIE Lab is extensively used in many industries. It's uses include providing exact colour specifications for paint (including automotive), dyes (including textiles, plastics, etc.), printing ink and paper. It is becoming of increasing importance in specifying printing standards such as in ISO-12647, where it is often used instead of densitometry. For example "Paper Type 1" (115gsm gloss coated white, woodfree) has "Paper Shade" described as L* 95, a* 0, b* 0. "Paper Type 5" (115gsm uncoated yellowish offset) is described as L* 90, a* 0, b* 9.  If you compare the different Lab values for Type 1 & 5 you will understand the descriptions. Lab measurements can be used to control printing, typically by monitoring a 3-colour neutral grey mid-tone patch.

In ICC Colour Management  CIE Lab is often used as the Profile Connection Space (PCS) where it provides a "link" between two colour profiles, such as Input RGB (scanner or camera) and Output (press or inkjet CMYK or RGB). All ICC profiles contain a PCS. In an input  profile the tables will convert the scanner's or camera's RGB space to the PCS (Lab). An output profile will convert the  PCS (Lab) to the digital printer's or printing press's colour space (CMYK). The other PCS colour space is CIE XYZ, but that is beyond the scope of this article; maybe later!